til/typescript/create-a-union-type-from-an-array.md

Create A Union Type From An Array

Let's say we have an array of actions that our program knows how to handle.

// inferred type: string[]
const actions = ['increase', 'decrease', 'reset'];

The inferred type of that array is string[] which is a pretty wide type. We can't do much with it. We can prevent the widening of this array's inferred type using as const.

// inferred type: readonly ['increase', 'decrease', 'reset']
const actions = ['increase', 'decrease', 'reset'] as const;

That inferred type is specific enough that we can do something with it, like create a union type.

const actions = ['increase', 'decrease', 'reset'] as const;

type Actions = typeof actions[number];
//=> type Actions = 'increase' | 'decrease' | 'reset'

We could use the Actions type to specify that a function only takes values that correspond to known actions, for instance.

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